We have previously demonstrated that a fertilization membrane forms around a sea urchin egg when it is injected with a soluble spermatozoa fraction isosmotic with seawater. We found that a spermatozoon contains inositol trisphosphate at such a high level as to trigger calcium elevation leading to the exocytosis of the cortical granules and the elevation of the fertilization membrane. We observed a similar amount of inositol triphoshate in goat sperm. Thus, inositol triphosphate plays the role not only of a second messenger within eggs as in other cells but of the primary messenger from spermatozoa to eggs at fertilization.